Excimer lasers are well known. One important use of excimer lasers is as a light source for integrated circuit lithography. One type of excimer laser currently being supplied in substantial numbers for integrated circuit lithography is the ArF laser which produces ultraviolet light at a wavelength of 193 nm. A similar excimer laser, the KrF laser, provides ultraviolet light at 248 nm. Both of these wavelengths are considered to reside in the deep ultraviolet (“DUV”) portion of the electromagnetic spectrum.
These lasers typically operate in a pulse mode. The laser beam is produced in a laser chamber containing a gain medium created by a discharge through a laser gas between two electrodes. For an ArF laser the laser gas is typically about 3 to 4% argon, 0.1% fluorine and 96 to 97% neon. For a KrF laser, the laser gas is typically about 1% krypton, 0.1% fluorine and about 99% neon.
Fluorine is the most reactive element, and it becomes even more reactive when ionized during an electric discharge. Special care must be exercised to utilize in these laser chambers materials such as nickel-coated aluminum which are reasonably compatible with fluorine. Further, laser chambers may be pretreated with fluorine to create passivation layers on the inside of the laser chamber walls. However, even with this special care, fluorine will react with the walls and other laser components producing metal fluoride contaminants and resulting in a relatively regular depletion of fluorine gas. The rates of depletion are dependent on many factors, but for a given laser at a particular time in its useful life, the rates of depletion depend primarily on the pulse rate and load factor if the laser is operating. If the laser is not operating, the depletion rate is substantially reduced. The rate of depletion is further reduced if the gas is not being circulated. To make up for this depletion, new fluorine or a gas mixture containing fluorine is typically injected at regular intervals. These and other details of the operation of these lasers can be found in U.S. Pat. No. 6,240,117, titled “Fluorine Control System with Fluorine Monitor” issued May 29, 2001, the entire disclosure of which is hereby incorporated by reference.
In some present systems, indirect measures of laser performance are used to estimate F2 consumption. Such indirect measures are generally effective to provide long term reliable operation of these excimer lasers in a manufacturing environment. However, various factors (changing operating point, contaminant generation) can lead to errors in the estimate, causing drift in performance over gas life and ultimately unacceptable error rates.
Direct measurement of F2 concentration in the gas would avoid these difficulties. Direct F2 measurement is possible with chemical sensors but these are typically slow and require large sample volumes (or continuous flow) of gas to establish an accurate reading. Sampling significant fractions of the gas in the chamber would increase overall consumption of gas and likely lead to changes in performance while sampling is taking place (i.e., the chamber pressure drops considerably when a F2 measurement is made). Additionally, frequent and time-consuming calibration is necessary.
There is therefore a need for an apparatus for and method of determining fluorine depletion in fluorine-based excimer lasers such as ArF and KrF excimer lasers. This need is especially acute in lasers having a dual chamber design and pulsed power architecture where the two chambers receive identical charge voltages thus rendering estimation of fluorine consumption more difficult.